Alarm systems such as fire alarm systems and/or break-in alarm systems usually have an alarm center including an attached local signal line network via which peripheral sensors are connected in certain areas of a building to be monitored, for example. These sensors are motion alarms or fire alarms, for example. Such security systems are predominantly designed according to digital alarm line technology.
For easy installation and low overall costs, the peripheral sensors and other network elements such as couplers are supplied with power from the alarm center via a two-wire line. The information is transmitted over this alarm line from the alarm center to the sensors via voltage-modulated signals, which are superimposed on the supply voltage. Information from the sensors to the alarm center is transmitted via current-modulated signals which are added to the supply current needed by the sensors. These current-impressed signals are detected by a current sensor element in the alarm center. In order to achieve simple signal analysis using this information, all alarms and couplers are designed as constant current sinks which always draw the same current within their allowed operating range, regardless of the voltage applied. During operation, the system is always operated using a signal-superimposed and therefore variable DC voltage applied to the peripheral elements, while a signal-superimposed and therefore variable DC current is flowing.
A configuration of a security system using digital alarm line technology disclosed in German Patent No.-100 48 599 is schematically shown as a highly simplified block diagram in FIG. 1.
Series-connected peripheral sensors 4, 6, 8, 12, and 14 are connected to alarm center 1 in a ring structure as a local security network (LSN).
An alarm center 1 has an alarm line terminal 2 and a power supply terminal 3 for the ring input, and an alarm line terminal 10 and a power supply terminal 11 for the ring output. Alarm line terminal 2 is connected to alarm line 16. In this example, alarm line 16 is the local security network (LSN) by Bosch.
Alarm line terminal 10 is also connected to alarm line 16. Power supply terminals 3 and 11 are connected to power supply line 17. Alarms 4, 6, 8, 12, and 14 are connected in series to alarm line 16. Power supply devices 5, 7, 13, and 15 are connected in series to power supply line 17, while alarm 8 may be supplied with power via alarm line 16. Simultaneous signal transmission and power supply over alarm line 16 are achieved by modulating both the supply voltage and the current. Alarm center 1 transmits data via pulse-length-coded modulation of the supply voltage to alarm 8. The alarm center also transmits data via pulse-length-coded information to alarms 4, 6, 12, and 14. In the case of multiple sensors 4, 6, 8, 12, and 14, alarm center 1 assigns successive digital addresses at which alarms 4, 6, 8, 12, and 14 are subsequently addressed via the appropriate pulse-length-coded voltage modulation. For a supply voltage of 30 V, the voltage range of these signals is approximately 1.6 V.
Alarm 8 supplied with power via alarm line 16 transmits its response signals, i.e., useful signals, to alarm center 1 via pulse-length-coded modulation of the current received. Alarms 4, 6, 12, 14 transmit their response signals, i.e., useful signals, to the alarm center also via pulse-length-coded modulation of the current received. These useful signals for alarm center 1 are coded as current peaks of approximately 10 mA, are picked up via a 5-ohm resistor used as a current sensor in alarm center 1 and, after conversion to digital voltage signals, supplied to a processor for analysis.
The required current intensity is determined as a function of the number and type of sensors 8, 4, 6, 12, and 14 connected to alarm line 16. In this exemplary embodiment corresponding to the related art, the maximum current intensity on alarm line 16 is 100 mA. This limits the number and power consumption of the alarms supplied with power exclusively via alarm line 16 as a function of the power consumption. In the related art usually only alarms having low power consumption, such as fire alarms, are supplied with power in this way. In contrast, alarms 4, 6, 12, and 14, which require more power, receive the power required by the sensor system via power supply line 17. These alarms receive the low power for the electronics which makes communication with the alarm center possible also via the alarm line.
In addition to systems having a ring structure, there are also alarm systems having alarms connected via spur lines.
In the known alarm systems, the current sensor element is usually designed as an ohmic resistor. The signal current intensities analyzable as signals equal approximately 10 mA. As long as a supply current or basic current of a maximum of 100 mA or 300 mA, respectively, is needed on the alarm line for supplying the peripheral network elements, the useful signal to noise ratio, e.g., the noise level, is kept within acceptable limits for reception.
However, if considerably higher supply currents or basic currents are required due to a correspondingly larger number of connected alarm, control, and signaling devices according to the present trend toward increased security via higher monitoring complexity, the useful signal to noise ratio becomes less favorable. For example, at a supply current intensity of 1.5 A, the noise in the receive amplifier circuit would reach a level at which the approximately 10 mA useful signal would be difficult or impossible to detect at an ohmic resistor. Such a signal current amplitude, however, is still highly desirable for these types of security systems for reasons of compatibility with existing central signal processing and signal analysis systems.
An excessively high power loss of P=R*I2=5 ohms*(1.5 A)2=11.25 watts would also occur across a 5-ohm resistor typically used for detection. Therefore, in presently known alarm systems, alarms and other network elements which would cause the current intensity to exceed 100 mA or 300 mA on the alarm line are supplied with power by an additional power supply line. This is associated with additional cost and complexity.
This signal reception problem may possibly be overcome by filtering out the entire DC current component using a frequency filter to subsequently analyze the AC current component. However, in doing so, the signal shape is disadvantageously affected in the case of large detection ranges, for example, between 1000 m to 3000 m, as required in larger alarm systems. In addition, the filter components are relatively expensive when designed for relatively high current intensities.